The invention relates to an electroluminescence device comprising a multilayer system including an active electroluminescence layer made of an organic semiconductor and capable of emitting light under the influence of an electric field, the electroluminescence layer disposed between a cathode layer for injecting electrons into the electroluminescence layer and an anode layer for injecting holes into the active layer, wherein, if necessary, a hole injection layer is disposed between the anode layer and the electroluminescence layer and/or an electron injection layer is disposed between the cathode layer and the electroluminescence layer.
Known devices of this type emit light in a spectral region mostly representing the region of the visible spectrum with the higher energy characterized mainly by the visible color blue and the invisible ultraviolet. Light in this spectral region has the major advantage that it can be converted by simple means into all the other colors in the visible spectrum. For example, a fluorescent dye may be excited which in turn emits light in a spectral region having a lower energy--i.e., at a longer wavelength--by way of photoluminescence.
The light in the active electroluminescence layer is generated by organic semiconductors, whereby the electrons and holes are injected into this organic semiconductor from opposing electrodes if an external voltage, usually between 5 and 20 Volts, is applied. Electroluminescence occurs when the electric carriers of opposite charge combine to form exciton states (singlet excitons) which subsequently decay radiatively. Organic electroluminescence devices can be designed to have a thickness in the order of microns, to be flexible and to cover large areas, while emitting colors from the near-infrared to the ultraviolet portion of the spectrum. The application of organic semiconductors for flat panel displays is very promising due to their high luminance (&gt;10,000 cd/m.sup.2), their high efficiency and their low operating voltages. Thin film organic electroluminescence devices have found applications in multi-color flat panel displays because they are simple to manufacture, have a high brightness and provide a large number of different emission colors.
The first scientific publication in the field of polymer semiconductor diodes was published by J. H. Burroughs in Nature 347 (1990) and relates to the use of polyphenylene-vinylene (PPV) as an active electroluminescence layer.
The emission of visible light from organic semiconductors is also known from an article in Appl. Phys. Lett. 51, (1987) describing the use of electroluminescence from conjugated polymers, such as MEH-PPV, for making a light-emitting diode (LED) having a MEH-PPV layer placed on a glass substrate coated with indium-tin-oxide (ITO) and a rectifying metal contact disposed on top of the MEH-PPV layer. In this article, the preferred explanation of light emission is based on a tunneling model whereby the electrons tunnel directly from the rectifying metal contact into an upper polaron level.
In the article Adv. Mat. 4 (1992), 36-37, the problem associated with the generation of blue light by a LED is discussed and a blue-emitting PPP-LED is proposed which includes the customary ITO glass substrate and the aluminum electrodes.
The application of double-stranded poly-(paraphenylene) LPPP with partially protonized side groups in electroluminescence devices was first reported by G. Grem and G. Leising in Synthetic Metals 1993.
From DE-A1-38 04 293 there is known an arrangement with an electroluminescence diode wherein the light-emitting surface of the diode is covered with an element made of plastic, the element comprising an additive of titaniumoxide and at least one fluorescent, color converting organic dye, such as polymethylmethacrylate.
In JP-A-4 276 171 (Abstracts of Japan Vol. 18, No. 419) there is further disclosed an organic electroluminescence element having a strong emission as a result of the use of a transparent substrate, of transparent electrodes and of a color filter made of BPF. The use of a filter provides a substantial narrowing of the emission spectrum.
In U.S. Pat. No. 5,408,109 there are described LED's made of soluble semiconducting polymers, such as MEH-PPV, which do not require thermal treatment for their polymerization.
Furthermore, in U.S. Pat. No. 4,885,211 there are also discussed various organic luminescent materials, with an emphasis on a particularly preferred LED design wherein a hole transport layer and an electron transport layer is disposed between two electrodes, the one electrode having a high work function and the other electrode having a low work function. The cathode has a layer made of several metals with at least one of these layers having a high work function.
The electroluminescence cell discussed in U.S. Pat. No. 4,539,507 also includes a hole injection layer and an electron transport layer, with the first layer proximate to a transparent metallic anode for injecting the holes.
Color picture elements (pixels) of which a flat panel display screen is composed, can be implemented by arranging next to each other blue, green and red organic light-emitting diodes (OLED), or by using LED's emitting at optical wavelengths which can be selected based on the applied voltage, the electric current, the operating temperature or other parameters.
The abstracts of the "IS&T 49th Annual Conference" describe an organic electroluminescence device combining a blue organic electroluminescence display with color converting materials. The color converting materials basically consist of an organic fluorescent dye capable of changing the emitted color from blue to green or red. The blue organic multi-layer electroluminescence device consists of an ITO anode, a hole injection layer, a hole transport layer, an emission layer (DPVBI), an electron transport layer, and a cathode of magnesium and aluminum. For the generation of multiple colors, red, green and blue subpixels are arranged on a glass substrate in a pattern, wherein the red and green color, respectively, is generated by a corresponding red or green dye layer disposed on top of the blue emission layer. Disposed thereupon is a protection layer, followed by the anode and all the remaining layers, including the blue emission layer and the cathode. This arrangement has the disadvantage that the red color has only a very low intensity due to the low spectral content of this color in the blue color emitted by the active layer.
U.S. Pat. No. 5,126,214 describes an electroluminescence device where the blue light is converted into red and green light through fluorescent dye layers. Red light is generated from the blue fundamental emission by superimposing two different dye layers. Electron transport layers as well as hole transport layers are provided for improving the efficiency of light generation.
In addition, U.S. Pat. No. 5,294,870 describes an electroluminescence device emitting in the blue with the pixel matrix of the display screen being patterned. The fundamental emission in the blue is converted through fluorescent dye layers into the other primary colors green and red, wherein each color pixel is subdivided into at least two subpixels.
The electroluminescence devices described above have the disadvantage that the optical efficiency is much too small for many applications.
A further disadvantage is that the emission spectrum of the known devices lies in an disadvantageous wavelength regime resulting in a rather weak intensity for color conversion.